[PATCH v2 0/1] remove SWAP_MAP_SHMEM

[Nhat Pham <nphamcs@gmail.com>]

Changelog:
v2:
	* Fix a WARN in the shmem THP swap path. Thanks Baolin, Yosry, and Barry
	for the report and the discussion on how to solve it.
	* Squash the two patches into one.
RFC v1: https://lore.kernel.org/all/20240923231142.4155415-1-nphamcs@gmail.com/

The SWAP_MAP_SHMEM state was originally introduced in the commit
aaa468653b4a ("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a
swap entry belongs to shmem during swapoff.

However, swapoff has since been rewritten drastically in the commit
b56a2d8af914 ("mm: rid swapoff of quadratic complexity"). Now
having swap count == SWAP_MAP_SHMEM value is basically the same as having
swap count == 1, and swap_shmem_alloc() behaves analogously to
swap_duplicate()
    
This RFC proposes the removal of this state and the associated helper to
simplify the state machine (both mentally and code-wise). We will also
have an extra state/special value that can be repurposed (for swap entries
that never gets re-duplicated).

Another motivation  is the new swap abstraction I am currently working on,
that would allow for swap/zswap decoupling, swapoff optimization, etc. The
fewer states and swap API functions there are, the simpler the conversion
will be.

Nhat Pham (1):
  swap: shmem: remove SWAP_MAP_SHMEM

 include/linux/swap.h | 16 ++++++++--------
 mm/shmem.c           |  2 +-
 mm/swapfile.c        | 41 +++++++++++++++++++++--------------------
 3 files changed, 30 insertions(+), 29 deletions(-)


base-commit: 391cad4424af8bb563e1504c5adaef0155b4abb6
-- 
2.43.5